The Global System for Mobile communications (GSM) system is a set of ETSI (European Telecommunications Standards Institute) standards specifying the infrastructure for a cellular terrestrial communications system. Specifically, referring to FIG. 1, the ground network of the GSM system is constituted by a number of mobile switching centers (MSC) 1b-4b with associated visitor location registers that act as gateways 1-4 between the mobile terminal and a public switched telephone network (PSTN) 7 or other terrestrial networks. A single logical home location register (HLR) 1c, 3c serves mobile users accessing service through multiple mobile switching centers.
A cellular satellite communications system, when integrated with the existing GSM system's infrastructure to form a satellite GSM system, utilizes the mobility, network interworking, and service management capabilities developed for the cellular terrestrial communication system. As illustrated in FIG. 1, a simple integration results in a basic satellite GSM system which uses the GSM cellular technology standard to provide the ground segment infrastructure for the cellular satellite communications system 8. In this basic satellite GSM system, since only the radio access has been extended through a satellite repeater, each of the mobile switching centers 1b, 2b, 3b, and 4b operating within the satellite coverage area 6 of the basic satellite GSM system 8 is assigned to a non-overlapping domain of coverage. Therefore, as indicated by the differently shaded areas in FIG. 1, location areas of the basic satellite GSM system 8 are defined according to the regional coverage assigned to a particular mobile switching center 1b-4b.
Additionally, each of the gateways 1, 2, 3 and 4 supports a satellite RF equipment and land earth station (LES) subsystems 1d, 2d, 3d, and 4d for accessing the space segment. System control is distributed among the mobile switching centers 1b, 2b, 3b, and 4b within the satellite coverage. The mobile switching centers incorporate associated visitor location registers (VLR) 1a, 2a, 3a and 4a. Additionally, home location registers 1c and 3c, as part of the GSM mobility management system, are implemented at one or more gateways 1 and 3 depending on the number of subnetworks supported.
As a mobile terminal moves from one cell to another cell within the satellite coverage 6 of the basic satellite GSM system, its location is updated and registered according to the location areas identified (i.e., via the system control channels). If the mobile terminal remains within the defined coverage of a particular gateway, its location is updated and registered only at that particular gateway. However, if the mobile terminal moves from the coverage area of one gateway to the coverage area of another gateway, the location of the mobile terminal is updated and registered at the new gateway, and a new serving visitor location register is conveyed to the home location register.
One of the problems in the basic satellite GSM system of FIG. 1 is that it does not utilize the extended coverage capability of the satellite. As a result, calls to and from the mobile terminal are not always completed through the most efficient path. Under the standard GSM procedures, calls to and from the mobile terminal are only completed through the gateway at which the terminal is currently registered. For instance, as illustrated in FIG. 2, a call from the fixed terminal 10 to the mobile terminal 18 must always be routed by terrestrial means (PSTNs) 11 and 14 to the gateway (MSC/VLR) 16 at which the mobile terminal is registered. Therefore, even if the call from the fixed terminal 10 is delivered to the one of the mobile switching centers (e.g., home gateway 13) within the coverage of the satellite 17, the call must be terrestrially routed to the mobile switching center (e.g., visiting gateway 16) at which the mobile terminal is registered because the standard GSM procedure 19 allows only the registered mobile switching center 16 to access the mobile terminal 18 directly.
For a mobile originated call to a fixed terminal, a similar routing inefficiency results. If the mobile terminal originates a call to a fixed terminal, the call is established through the registered mobile switching center. For instance, even if a call destined to a fixed terminal is connected to a mobile switching center other than the registered mobile switching center, the call connection is not established through the mobile switching center at the gateway which is nearest to the fixed user. Instead, the mobile terminal completes the call through the registered gateway from which it is further routed terrestrially to the destined fixed terminal. For instance, as illustrated in FIG. 2, a call from the mobile terminal 18 to the fixed terminal 10, instead of directly routed from the mobile terminal 18 to the gateway 13 via the satellite 17, is routed from the mobile terminal 18 to the gateway 16 via the satellite 19, and then to the gateway 13 via terrestrial means. In short, although the satellite in the cellular satellite communications system has a wide coverage area, the mobility management procedures of the basic satellite GSM system prevent this capability from being utilized. As a result, the least cost routing for the mobile terminal originated and terminated calls is not achieved.